Duo Workflow Executor: Pass tokens in gRPC metadata to duo-workflow-service

On startup we'll need to pass 2 tokens through to the Duo Workflow Service in the gRPC connection:

1. JWT for Cloud Connector auth
2. GitLab OAuth token

Read about this auth in gitlab-org/gitlab#470128 (closed) and read about gRPC metadata in https://grpc.io/docs/guides/metadata/

We should update the executor to allow these tokens to be passed in as environment variables (perhaps as well as command line arguments).

You can see how the Cloud Connector token should be passed to the Duo Workflow Service in duo-workflow-service!12 (merged) .